1. Field of the Invention
The invention relates to detecting a level of an amplified signal. More particularly, the invention relates to detecting signal levels in a communications transmitter or receiver.
2. Background Information
A power detector is a circuit that receives a time-varying voltage as an input signal and outputs an indication of the power level of the input signal. For example, a power detector may output a DC voltage signal that has a voltage level proportional to the power of the input signal. A power detector may be used to detect the highest peak value or all peak values of the input signal.
Power detectors are often used to monitor power amplifier outputs. One common use of power detectors is to facilitate gain adjustments in automatic gain control amplifiers. A power detector used in this capacity monitors the signal outputted by the amplifier and indicates the power level. The power level indication is used to adjust the amplifier""s gain accordingly. Such control may help to minimize the effects of undesirable changes in the amplifier""s output signal level.
One area in which power detector circuits are particularly useful is in the field of wireless communications. For example, a power detector may be used in a code-division multiple-access (CDMA) wireless phone system to ensure that the maximum transmitting power requirements are not exceeded. During transmission, the power detector converts the output power of a transmitting power amplifier into a DC voltage, which may be used by other circuits to determine and/or adjust the transmitting power.
In practice, power detector circuits are used in a number of different capacities and are available in a variety of different configurations. One method of power detection uses the nonlinear characteristics of a PN junction diode to detect the peak voltage values of an input signal. Unfortunately, this method produces inaccurate output results because the diode characteristics are highly sensitive to temperature variations. Even minor changes in the power detector""s operating environment may have a significant impact on the accuracy of its output. Consequently, for a certain input signal power, changes in the operating temperature of a power detector""s circuit may alter its output signal by up to 50% or more. Stated differently, a single input signal having a known input voltage level will produce output signals having significantly different voltage levels depending upon the temperature of the power detector circuit.
FIG. 1 is a diagram of a RF transmitter section 2 that may be fabricated on a circuit board for use in a wireless communications device (e.g. a cellular telephone). Transmitter section 2 includes a discrete power detector circuit 4, a power amplifier 6, and an antenna 8. An output of the power amplifier 6 is applied to an input of the discrete power detector 4 through resistor-capacitor (RC) coupling. Diode D1 detects a power level of the input signal, and the discrete power detector circuit 4 provides an output based on this indication at terminal Vout. Diode D2, which is identical to diode D1, provides some measure of compensation for the temperature-varying response of diode D1.
In this example, power detector 4 is constructed from numerous discrete circuit components including resistors, capacitors, inductors, diodes, and an operational amplifier. The requirement of a large number of discrete components increases system assembly costs and may also prevent the use of such a detector in an environment where circuit board space is restricted. Because of the inductive and capacitive values required by such a circuit, integration of discrete power detector 4 is not feasible. Additionally, this discrete method of power detection often requires additional temperature compensation and sophisticated calibration and software to obtain the accuracy and dynamic range needed for an application.
As stated above, power detector circuits are commonly used to monitor the output signal levels of power amplifiers. However, discrete power detectors require a large number of discrete components on the circuit board and therefore increase the system cost. In addition, existing power detectors demonstrate poor detection accuracy due to temperature variations and therefore require complicated calibration, also increasing the system cost. Additionally, the operating temperature of a power amplifier may be expected to vary over a range greater than 100xc2x0 C. For such reasons, it has not been feasible to fabricate a power amplifier and a power detector on the same integrated circuit.
Given the constraints of size in modern electronics components, there is a need for a power detector and a power amplifier formed on a single IC. Further, there is also a need for a power detector capable of compensating for the effects of temperature variations.
Consistent with the principles of the present invention as embodied and broadly described herein, an integrated circuit according to one embodiment of the invention includes a power amplifier and a power detector coupled to the power amplifier. The power amplifier is configured and arranged to receive an input signal and to produce an amplified signal based on the input signal. The power detector is configured and arranged to detect a power level of the amplified signal. In this embodiment, the power detector is also configured and arranged to output a temperature-compensated indication of the power level. As described in particular embodiments as set forth herein, such temperature compensation may include receiving one or more compensation currents that are substantially proportional to absolute temperature at least over an operating temperature range of the integrated circuit.